This invention concerns the removal of gases from water, in particular the dearation of seawater which will be injected into deep structures in order to increase the extractable fraction of subsurface hydrocarbon resources.
In this connection it is important to remove the oxygen from the seawater in order to reduce corrosion rates and prevent growth of aerobic bacteria which will impede the flow of hydrocarbons from the structure.
This problem has been known for a long time and different solutions have been proposed. The great majority are based on one of two principles, degassing by reduced pressure, degassing by gas-stripping or a combination of these two. Large quantities of seawater are injected, and the degassing has to be satisfactory at all times. Untreated seawater is usually saturated with oxygen and it may cause severe corrosion if the deoxygenation treatment is not working satisfactory.
It is therefore necessary to put great demands on the quality of the deoxygenation process, and thus large and expensive plants are required.
Vacuum degassing is often considered unsuitable because of the very complicated and heavy equipment. Stripping with natural gas has therefore often been preferred in practice. Natural gas is easily accessible in great quantities offshore and has so far been used as a stripping agent for deoxygenation. After the stripping process, however, the gas is not suitable for sale and has to be flashed.
Pollutants in the natural gas, including CO.sub.2 and H.sub.2 S, reduce the quality of the water and this is also a disadvantage. Furthermore, stripping towers using natural gas have a high gas consumption, and the weight and volume are almost the same as for vacuum towers.
A stripping plant employing recirculated nitrogen gas is described in U.S. Pat. No. 4,017,276 (Norwegian patent Application No. 77 2185). According to this patent, deoxygenation is performed in a stripping tower using nitrogen gas. The oxygen gas is removed from the nitrogen using low temperature fractionation. The gas consumption hence will be low.
The fractionation device however, is bulky and expensive, and the cooling device consumes large amounts of energy. Expensive precautions are required in order to obtain sufficiently low oxygen levels. Nitrogen stripping gas from gas fractionating generally contains 10 ppm to about 100 ppm oxygen and must be further purified before recycling.
Seawater used for injection is usually lifted from below sea level by submersible centrifugal pumps. It has been proposed to use a gas-lift instead. This is a simple system. It normally has a higher reliability than mechanical pumps, and it is capable of pumping the water to elevations high above the surface.
The gas-lift pipes will under certain conditions have a stripping effect, causing removal of oxygen from the water. Natural gas has been used in gas-lift systems. A great drawback, however, is the demand for great amounts of natural gas. The consumption is at least three times higher than for stripping towers. Furthermore, H.sub.2 S and CO.sub.2 will be transferred to the water and acidify it.